The present invention relates to:
a) a process for the fabrication of a structural element, in particular, an optical structural element; PA0 b) various types of optical elements; PA0 c) an optical imaging system; PA0 d) a vacuum treatment installation for the fabrication of an optical element; PA0 e) a process for tracking the erosion of material or the deposition of material; PA0 f) an installation for such tracking; PA0 g) a process for determining the point at which a surface has been reached during a reactive etching operation; PA0 h) an etching process control method; and PA0 i) a stop layer for an etching process.
Although the process according to a), and, consequently, also the vacuum treatment installation according to d), are suitable for the fabrication of a broad spectrum of structural elements each comprising a carrier substrate as well as at least one dielectric layer which, at at least one area, is to be eroded down to a given thickness, the gist or essence of the present invention is that it responds essentially to the requirements of semiconductor fabrication technology.
The structuring of dielectric layers, like structuring of metallic layers, is an essential process step in semiconductor technology. For the removal of dielectric layers of this type various methods have been used.
A first method, known as the lift-off technique, comprises the application of a photosensitive material, such as photoresist, to the substrate, exposing the material or resist to a desired pattern, developing the resist and then cleaning it. Depending on whether or not a positive or a negative resist has been used, the non-exposed or the exposed areas of the resist remain. A layer system is applied onto the substrate treated in this way, and subsequently the photosensitive resist disposed under the layers is dissolved by using an appropriate solvent. In this way, the layer system is removed in those regions of the substrate where the resist was remained. It is essential that the layer system does not impermeably close off the resist against the outside, especially at the edges of the region, in order not to preclude penetration by the solvent.
A second method comprises first applying the layer system onto the substrate. Subsequently the photosensitive resist is applied, exposed to the desired pattern, and developed. The layer system is thereby exposed at those sites or areas at which partial removal or removal down to the substrate is to take place. The latter takes place through bombardment by means of noble gas ions at typical energy values of 1000 eV and a typical ion current density of approximately 1 mA/cm.sup.2. The layer material is thereby etched away as well as the photosensitive resist. Since the etching rate of the resist is, in general, higher than that of the layer system, a thick resist has to be applied. This is done in order to prevent the resist from being etched away before the sites which are not covered by resist are removed down to the desired depth.
This process is also referred to as "ion milling" and is not selective in the sense that the etching rates for layer materials of the same type, such as for example of metal oxides, are not significantly different. An advantage of this ion milling method consequently resides in that it is not a process specific to the material to be etched.
A third method is reactive etching (RE). Departing from a layer system using a mask, for example made of a photosensitive resist as in ion milling, a gas is activated selectively, depending on the layer material to be removed, in the sense that reactive gas particles are generated which convert the layer material, which through the mask is exposed at particular sites, into volatile reaction products which are subsequently pumped off. In this way the layer system is removed or eroded. Through a suitable selection of the activated gas, called in the following a "reactive gas", it can be achieved that only a particular material is selectively and considerable etched whereby a high selectivity is achieved. By choosing a high selectivity with respect to the mask material, for example with respect to a photosensitive resist, only a thin layer of it need be applied. Also, the etching rates which can be achieved in reactive etching are be greater by decades than the rates achieved by ion milling. Consequently, this third method is in general economically more advantageous than is the ion milling process.
Activation of the reactive gas can take place in different ways, for example directly on the surface to be removed through laser beam bombardment or spatially distributed through laser beam effect, microwave energy or through ion or electron beams. Subsequently, the reactive gas activation can take place in a glow discharge whereby reactive species are formed.
While reactive etching through local laser effect can lead to high thermal loading of the layer system, those processes in which over the layer surface to be eroded an homogeneous density distribution of the reactive gas species is achieved, as is the case especially by means of a glow discharge, have the further essential advantage compared to the ion milling process that the edge profiles of the etched surface areas can be controlled better in the sense that practically ideal vertical steps can be achieved in the structure, if desired.
It is known from U.S. Pat. No. 4,684,436, to apply a pattern by means of a laser ablation process onto the surface of a work piece, there the intensity of a laser beam use during the process is modulated by means of a mask with a locally different layer system. The mask comprises a dielectric layer system on which, for achieving different energy transmission values through the above stated ion milling process, regions are etched to a greater or lesser depth or a number of the provided layers is selectively etched off. With respect to the layer stack structures of a mask of this type, which structures, as will become evident, can also be realized according to the present invention, U.S. Pat. No. 4,684,436 is incorporated here by reference.
It is further known from U.S. Pat. No. 4,923,772, to use excimer lasers for laser ablation processes, for example operating at a wavelength of 248 nm and wherein for the ablation process energy densities &gt;100 mJ/cm.sup.2 are required, and to use as a mask layer system with a highly reflecting dielectric layer stack which is stable with respect to high beam energy flows (laser damage threshold). The mask is made of a multi-layer stack, alternatingly with layers of high and low refractive index materials. As the material with high refractive, hafnium oxide, scandium oxide, aluminum oxide or thallium fluoride is suggested. The surface removal on the mask layer stack is said to be realizable through an ion milling process or through glow discharge or through reactive ion etching, whereby, however, the glow discharge (plasma) or reactive ion etching is said to be slower and more difficult in the case of dielectric layers, due to the fact that the high-refraction material tends not to be reactive. Therefore, according to U.S. Pat. No. 4,923,772, the suggested layers of high refractive index material are structured by using ion milling or the lift-off technique.